[CT-2115] In-repo documentation: glossary of terms

[jtcohen6]

These words all have specific meanings, whether referring to an internal process or object of importance to dbt-core:

• parsing
  • rendering, contexts, macro_manifest
  • which contexts are available where
  • "late-rendering" of certain configs (pre/post-hook)
  • node parsing / construction
  • resolution (references)
• compilation
  • "introspective" queries
• execution / materialization
  • "mutative" queries
• manifest
  • files / partial parsing
• graph
• node (manifest object)
• catalog
• results
• adapter
• relation
• cache

Let's develop a single place where we maintain up-to-date definitions of all these terms. These are all API objects, so I think this codebase is the right place to maintain the glossary (versus end-user documentation at docs.getdbt.com). The target audience is dbt-core developers, contributors, and anyone curious enough to wade into the technical details. Not relevant to the vast majority of end users. Other potential names for this: "field notes," "practitioner's guide," ...

Potential inspiration:

• Parsing vs. Compilation vs. Runtime (public)
• Parsing vs Compilation vs Runtime (internal Notion doc)
• The Adapter Cache (internal Notion doc)
• Of caches & catalogs (internal Notion doc)